•The secret to cool bodies and glazes is a lot of testing.
•The secret to know what to test is material and chemistry knowledge.
•The secret to learning from testing is documentation.
•The place to test, do the chemistry and document is an account at https://insight-live.com
•The place to get the knowledge is https://digitalfire.com

A white very light-weight powder (and bag of this material is like a bag of feathers). It is practically insoluble in water. This is not the same material as magnesium carbonate (magnesite).

This material is very refractory, it does not decompose and give up its MgO to the glaze melt easily. Magnesium carbonate is a good example of the need to consider mineralogy and material level physics in addition to chemical makeup when using ceramic chemistry to adjust and fix glazes. Two glazes may have the same calculated chemistry, but the one using the mag carg to source the MgO will not be melted as much.

When added to glazes magnesium carbonate often mattes them, simply because it is refractory (it is useful at lower temperatures where making a good matte can be quite difficult). Even it is does decompose into the glaze melt, the MgO it supplies is a classic matting mechanism. Magnesium carbonate can also induce crawling because of the decreased mobility of the glaze melt and the exceedingly small particle size of the material increases shrinkage.

This material has a very high Loss on Ignition, this could cause glaze surface issues. Dolomite and talc more readily release their MgO to the glaze melt for a higher temperature glazes. There is conflicting information on the decomposition temperature, sources report figures that vary from 350C to 650C. Thus it is possible that a host glaze could be already fluid before this material has finished gassing, trapping it within to create the glass equivalent of an Aero chocolate bar. If you use a large percentage of this in a glaze (for crawling for example), it may be wise to halt the kiln at 650C and soak during the fire up. Or do a test firing to 700C, cool the kiln quickly and check the melted state of the glaze.

It is added (.12-.25%) to flocculate clay slurries, improving and stabilizing the set and suspension characteristics.
It is used as a electrolyte in both ground and cover coat enamels.

An additive that will make a glaze crawl

Light magnesium carbonate has been added to a low temperature terra cotta white glaze (about 10%). It induces crawling. It also mattes the glaze because it sources MgO.

Ceramic materials can vary widely in density

A bag of magnesium carbonate beside a bag of feldspar. Although the former weighs 25 kg (vs. 22.7 kg for the feldspar), clearly it is a dramatically lighter (per volume unit) material. Lifting that bag of Mag Carb feels like lifting a pillow!

Magnesium carbonate vs. oxide: One big difference

Here is a screenshot of side-by-side recipes in my account at insight-live.com. It takes 120 mag carb to source the same amount of MgO as 50 mag ox. I just made the two recipes, went into calculation mode and kept bumping up the magcarb by 5 until the chemistry was the same. Note the LOI of the magcarb version is 40. This one would certainly crawl very badly.

LOI is not important? Think again!

This chart compares the gassing behavior of 6 materials (5 of which are very common in ceramic glazes) as they are fired from 500-1700F. It is a reminder that some late gassers overlap early melters. The LOI (loss on ignition) of these materials can affect your glazes (e.g. bubbles, blisters, pinholes, crawling). Notice that talc is not finished until after 1650F (many glazes have already begin melting by then).

Mag carb causes crawling in higher amounts

Example of two crawling glazes. Both have magnesium carbonate added to make this happen (around 10%). On the left at cone 04 on a terra cotta body, on the right at cone 6 on a porcelain. Magnesium carbonate also mattes glazes.

A condition where glaze separates into clumps or islands (during firing) leaving bare clay patches showing in-between.
This problem is almost always caused by glazes shrinking too much during drying and then cracking. Those cracks become the crawl-points during firing. Excessive shrinkage is norm...

A glaze that is not glossy. Of course, unmelted glazes will not be glossy, but to be a true matte a glaze must be melted and still not glossy. To be a functional matte it must also resist cultery marking, clean well and not leach into food and drink. Thus it is not easy to make a good matte glaze. I...